The present invention relates to a control device of an engine, and particularly to a control device of an engine which is operated at a high compression ratio.
Generally, for engines using gasoline or a fuel mainly composed of gasoline, a spark-ignition method in which ignition is performed by an ignition plug is broadly adopted. Recently, arts for performing compression self-ignition (specifically, premixed compression self-ignition referred to as HCCI (Homogeneous-Charge Compression Ignition)) within a predetermined operating range while using gasoline or a fuel mainly composed of gasoline by applying a high compression ratio (e.g., 17:1 or higher) as a geometric compression ratio of the engine are developed in view of improving fuel consumption.
One art regarding an engine which performs such compression self-ignition is disclosed in JP2012-172662A, for example. In the art of JP2012-172662A, the engine performs the compression self-ignition within a low engine load range and performs spark ignition within a high engine load range, and a period of time from a fuel injection start timing to an ignition timing is shortened by increasing a fuel pressure (i.e., a response time until ignition is shortened by retarding the fuel injection start timing), so as to suppress abnormal combustion, such as pre-ignition (a phenomenon in which mixture gas self-ignites before a normal combustion start timing triggered by spark ignition).
However, in the art of JP2012-172662A described above, the fuel pressure is increased to retard the fuel injection start timing as much as possible to the extent that the abnormal combustion (e.g., pre-ignition) does not occur; however in this case, the time period from the fuel injection until the ignition becomes excessively short, a sufficient mixing performance of the fuel (mixture gas) inside a combustion chamber cannot be secured, and smoke (soot) may be produced. Especially when an effective compression ratio is high, since it is desirable to greatly retard the fuel injection start timing to suppress pre-ignition, the time period from the fuel injection to the ignition becomes extremely short, and the possibility of producing smoke becomes high. Additionally, in view of the fuel consumption, when only the ignition timing is changed (i.e., an engine speed, an air-fuel ratio, etc., are not changed), it is desirable to apply a predetermined ignition timing with which a highest engine torque is obtained (generally referred to as “MBT” (Minimum advance for the Best Torque)); however, if such MBT is constantly applied, when the effective compression ratio becomes high, the time period from the fuel injection to the ignition becomes excessively short, and the possibility of producing smoke increases.